1. Field of the Invention
This invention relates to a focus detecting system in which scattered distance measuring points are disposed densely over a wide range in the photo-taking picture plane.
2. Related Background Art
As one type of the focus adjusting apparatus of a camera, there is known a method whereby the exit pupil of a photo-taking lens is divided into two by a focus detecting optical system, two object images formed by light fluxes passed through the pupil areas are received by photoelectric element arrays (for example, CCD sensor arrays), the focus state of the photo-taking lens is detected from the output thereof and the photo-taking lens is driven on the basis of the result of the detection.
Referring to FIG. 4 of the accompanying drawings, a field lens FLD is disposed coaxially with a photo-taking lens to be focus-detected. Two secondary imaging lenses FCLA and FCLB are disposed at positions rearward of the field lens and are symmetrical with respect to the optic axis. Further, sensor arrays SAA and SAB are disposed rearwardly of the secondary imaging lenses. Stops DIA and DIB are provided near the secondary imaging lenses FCLA and FCLB. The field lens FLD substantially images the exit pupil of the photo-taking lens LNS on the pupil surfaces of the two secondary imaging lenses FCLA and FCLB. As a result, light fluxes entering the secondary imaging lenses FCLA and FCLB, respectively, emerge from areas of equal dimensions on the exit pupil surface of the photo-taking lens LNS which correspond to the secondary imaging lenses FCLA and FCLB and which do not overlap each other. When an aerial image formed near the field lens FLD is re-imaged on the surfaces of the sensor arrays SAA and SAB by the secondary imaging lenses FCLA and FCLB, the two images on the sensor arrays SAA and SAB change their positions on the basis of the displacement of the position of the aerial image in the direction of the optic axis. Accordingly, if the amount of displacement (deviation) of the relative position of the two images on the sensor arrays is detected, the focus state of the photo-taking lens LNS can be known.
FIG. 5 of the accompanying drawings show an example of the photoelectric converted outputs of the two images formed on the sensor arrays SAA and SAB. The output of the sensor array SAA is A(i). and the output of the sensor array SAB is B(i). The number of picture elements of each sensor is at least five, and may desirably be several tens or more.
A signal processing method for detecting the amount of image deviation PR from the image signals A(i) and B(i) is disclosed by the assignee in Japanese Laid-Open Patent Application No. 58-142306, Japanese Laid-Open Patent Application No. 59-107313, Japanese Laid-Open Patent Application No. 60-101513 or Japanese Patent Application No. 61-160824.
By effecting the focus adjustment of the photo-taking lens on the basis of the amount of image deviation obtained by the method disclosed in these patent applications, the photo-taking lens can be brought into the in-focus state.
The method disclosed in the above-mentioned publications is such that for example, for the two image signals A(i) and B(i) (i=1, 2, . . . , N), ##EQU1## is calculated with respect to an integer value m. The range of i in which the sum is taken is determined from the condition that the suffixes i, i+k-m, i+k and i-m must be within a closed section [1,N]. k is an integer constant and usually, k=1. The range of m is concerned with the magnitude of the amount of image deviation to be detected and is not unconditionally determined, but usually m is varied within ##EQU2## The amount of correlation defined by the equation (1) is an example and the following discussion is also equally applicable to the other known amount of correlation than this. Besides the equation (1), there are the following formulas as expressions representing the amount of correlation: EQU .SIGMA.min{A(i),B(i+k-m)}-.SIGMA.min{A(i+k),B(i-m)} EQU .SIGMA..vertline.A(i)-B(i+k-m).vertline.-.SIGMA..vertline.A(i+k)-B(i-m).ver tline. EQU .SIGMA..vertline.A(i)-B(+k-m).vertline..sup.2 -.SIGMA..vertline.A(+k)-B(i-m).vertline..sup.2
The typical result obtained by calculating the above equation (1) with respect to each m is as shown in FIG. 6 of the accompanying drawings wherein the m at which V(m) reverses its sign is the amount of image deviation expressed in picture element pitch units. Usually this value does not assume an integer. Assuming that reversal of the sign has occurred between V(m.sub.O) and V(m.sub.O +1), the amount of image deviation M.sub.O including a fraction can be calculated by EQU M.sub.o =m.sub.o +.vertline.V(m.sub.o)/{V(m.sub.o +1)-V(m.sub.o)}.vertline..
A camera provided with a focus detecting apparatus of this type effects automatic focus detection for a distance measuring point usually placed like a spot in the central portion of the photo-taking picture plane. For example, in a single-lens reflex camera using 36 mm film, the length of the distance measuring field on the film-equivalent surface is usually of the order of 3-4 millimeters or less. The present invention relates to a focus detecting apparatus in which the field of view capable of detecting the in-focus condition is wider and a portion thereof is designated in the fashion of a spot by a manual or automatic method and the in-focus condition is detected for the designated position in the picture plane.
The arrangement as described below is conceivable as the arrangement of the optical system or the sensor of a focus detecting apparatus having a wide distance measurement range. For example, it may be a system in which the field lens FLD and subsequent elements of the focus detecting optical system shown in FIG. 4 are generally enlarged. In this case, there is provided in advance a long distance measurement range as indicated by 43 in FIG. 7B of the accompanying drawings and an effective field length 44 for which the in-focus is calculated is selected from it. Detection of the in-focus condition is effected by the output of the photoelectric sensor corresponding to the same field length 44. Usually, the distance measurement range 43 includes the background other than the main object intended by the photographer and therefore, it is meaningless to calculate the in-focus condition to effect focus detection for the entire range 43 by the use of the output of the whole photoelectric sensor, and it is also disadvantageous because the amount of calculation is enormous. FIG. 7A of the accompanying drawings shows the distance measuring field length 42 usually used in a prior-art automatic focus detection single-lens reflex camera relative to the picture plane size 41.
There can also be constructed a focus detecting optical system which has a plurality of the apparatuses of FIG. 1 in which the field length is conventionally set relatively short and in which the respective apparatuses correspond to different fields of view in the picture plane. Such an optical system is disclosed in Japanese Laid-Open Patent Application Nos. 62-47612 and 62-189415.
On the other hand, the focus detecting optical system shown in FIG. 8 of the accompanying drawings is the inventor's tentative plan which has five distance measuring fields 51a, 51b, . . . , 51e in the picture plane 50 lying on a predetermined imaging plane and these fields are separately imaged by different secondary imaging lenses 52a, 53a and others, for example, the field 51a is separately imaged on sensor arrays 54a and 55a by the secondary imaging lenses 52a and 53a. If there is automatic or manual means for selecting any one of the different fields, there will be provided a focus detecting apparatus for designating the detected in-focus position from a wide range in the picture plane.
If an apparatus for selecting the distance measuring point from a wide picture plane range is constructed of the focus detecting optical system as described above, there will arise a great problem when carrying it in the interior of a compact optical instrument such as a portable single-lens reflex camera. For example, when an attempt is made to enlarge the field length of FIG. 7A, it is necessary that secondary imaging lenses maintain a sufficient imaging performance for a wide angle field of view. The optical system of a focus detecting system carried in the interior of a portable camera is very small and therefore, it is difficult to adopt a complex lens construction to thereby widen the angle of view. Also, if the optical path length is made greater, the secondary imaging angle of view for the same field length is decreased, but again in this case, the total volume of the secondary imaging system is increased correspondingly to the increase in the optical path length, and this tends to be against the requirement for compactness. On the other hand, the apparatus of FIG. 8 requires adjustment of each of the juxtaposed secondary imaging systems and becomes very costly. Such an apparatus is difficult to make compact if adjusting members are individually added. In the heretofore known technique, there are a few requisite portions to be adjusted in a pupil divided image deviation detection focus detecting apparatus of the secondary imaging type, and it is incompatible with the requirement for compactness to carry the apparatus of FIG. 8 in into a camera.
The adjustment is already known or discussed in the patent publication proposed by the assignee and therefore need not be described in detail, but the adjustment required for two pairs of sensor arrays to see the same area of the predetermined imaging plane is most delicate and requires adjustment accuracy of the order of 1 .mu.m. The angle of tilt and the positioning of the optical member also unavoidably become considerably delicate because the entire system is small.
For such reasons, there has been desired a simple and compact focus detecting optical system which can direct object images in a wide range to the surface of the photoelectric sensor in a good imaged condition.